In recent years, input devices employing a so-called touch panel system, namely touch-type input devices have greatly increased. Not only for automatic teller machines (ATM) at banks and ticket-vending machines at stations, but also for portable phones, portable game machines and portable music players, the touch panel system has been increasingly employed as an input interface in association with the development in the thin display art.
A majority of touch panels that are currently used are based on the resistance film system or the electrical capacitance system, and besides these, those based on the optical system, electromagnetic induction system, and those utilizing surface acoustic waves by piezoelectricity are known. Usually, positional information is detected by means of these systems. To be more specific, the position where the operator touches (presses) on the touch panel is acquired as coordinate information, and a specified process is executed based on this information. As represented by a bank ATM, an operator of the device is able to operate it as if he/she pressed an actual button by touching the part of a button displayed on the screen. As a result of recent development in graphic user interface (GUI) processing technique, there is also known a device that enables a user to scroll the displayed image by stroking the screen, or to directly control a graphically-displayed slide switch with a finger.
In terminal information devices represented by “iPhone (registered trade name)”, a so-called multi-touch system in which an operation is made with two fingers is becoming mainstream in recent years. In the time to come, further diversity will be required for touch panels, and in recent years, the request to obtain information of pressing force concurrently with positional information is increased. To be more specific, if two kinds of information regarding what position on the screen is touched by the operator with what strength can be detected, the operability will be further improved.
As an art regarding this, Japanese Patent Laying-Open No. 5-61592 (Patent Literature 1) discloses the technique of concurrently detecting positional information and pressing force information by overlapping a position detecting device and a pressure-sensitive sensor.
Japanese Patent Laying-Open No. 2006-163618 (Patent Literature 2) discloses a system of acquiring pressing force information using a piezoelectric sheet while acquiring positional information by detecting at what part the detected voltage appears in plural electrode wires formed in a grid form in the piezoelectric sheet.
However, in the touch panel described in Patent Literature 1, on the ordinary touch panel that detects only position, a pressure-sensitive sensor formed from a piezoelectric sheet or a pressure-sensitive resistor sheet is overlapped. This pressure-sensitive sensor covers the entire surface of the touch panel.
An ordinary touch panel is usually installed on some image display device, and is requested to have high transparency. Each of the position detecting touch panel and the pressure-sensitive sensor has a plurality of films and electrode layers. With this system, it is impossible to detect multi touches. Although the entirety can be made transparent by making the films transparent and using a transparent conductive material such as indium tin oxide (ITO) for the electrode layers, there is still a problem that the light transmittance is impaired because of the large number of stacked layers. Further, necessity of a large number of parts and processes leads a cost rise. Further, since the positional information and the pressing force information are separately detected, there is a problem that the signal processing is complicated.
On the other hand, in the touch panel described in Patent Literature 2, the piezoelectric sheet is formed with a micro-wired electrode in a grid form for the purpose of concurrently detecting positional information and pressing force information. Since the positional information is obtained based on from what electrode in the grid electrode the signal is strongly detected, it is necessary to connect all of these micro-wires to an operation processing unit, and there is a problem that the structure is considerably complicated.
Japanese Patent Laying-Open No. 2006-39667 (Patent Literature 3) describes the art that enables multi-touch in a resistant film-type touch panel.
In any of those described in Patent Literatures 1 to 3, a transparent electrode of ITO or the like is used. ITO contains indium which is rare metal, and is susceptible to price increase due to depletion. Although transparent electrodes made of materials other than ITO are known, it requires a lot of work for providing the entire resin film with an electrode, and some deterioration in transparency is caused because transmittance is not 100% even with a transparent electrode.
Generally, with a mere resistant film type touch panel, not only multi-touch cannot be detected, but also pressing force cannot be detected. It also has such a problem that a transparent electrode of wide area should be used.
Generally, with a mere electrostatic type touch panel, although multi-touch is enabled, a touch with a pen cannot be applied and pressing force cannot be detected. Further, it has a problem that a transparent electrode of wide area should be used.    PTL 1: Japanese Patent Laying-Open No. 5-61592    PTL 2: Japanese Patent Laying-Open No. 2006-163618    PTL 3: Japanese Patent Laying-Open No. 2006-39667